Raising Exceptions and Custom Errors

15 min · Beginner

When your code detects invalid inputs or broken business rules, do not return dummy values like -1 or False. Raise an exception to halt execution and signal the failure explicitly.

Raising exceptions with raise

Raising an exception means creating an error on purpose with the raise keyword, to stop code that received invalid data.

Use the raise keyword followed by an exception class and an informative error message.

Python

1def set_discount(percentage):2    if not (0 <= percentage <= 100):3        raise ValueError(f"Invalid discount: {percentage}. Must be 0-100.")4    return percentage / 1005
6print(set_discount(15))

Example 1 · Rejecting a bad discount

Python

1def set_discount(percent):2    if not 0 <= percent <= 100:3        raise ValueError("Discount must be between 0 and 100")4    return percent5
6try:7    set_discount(150)8except ValueError as err:9    print(f"Rejected: {err}")

Output

Rejected: Discount must be between 0 and 100

What this code does

The function checks its input and raises ValueError when it is outside 0 to 100.
The caller catches it and prints the message.

Calling set_discount(150) immediately raises a ValueError with our clear explanation.

Defining custom exception classes

Custom exception is your own error type, a class that inherits from Exception. A clear name tells the reader which rule was broken.

Create your own custom exception by creating a class that inherits from Exception.

Python

1class InsufficientFundsError(Exception):2    """Raised when account balance is less than withdrawal amount."""3    pass4
5def withdraw(balance, amount):6    if amount > balance:7        raise InsufficientFundsError(f"Cannot withdraw {amount} from {balance}")8    return balance - amount

Example 2 · A custom error for a missing user

Python

1class UserNotFoundError(Exception):2    pass3
4def find_user(user_id, database):5    if user_id not in database:6        raise UserNotFoundError(f"User with ID {user_id} does not exist")7    return database[user_id]8
9db = {101: "Aditi", 102: "Kabir"}10try:11    user = find_user(999, db)12except UserNotFoundError as err:13    print(f"Handled: {err}")

Output

Handled: User with ID 999 does not exist

What this code does

Defines a custom UserNotFoundError.
Raises error when user ID 999 is missing from database.
Catches domain error and outputs clean message.

Custom exceptions allow API callers to catch specific domain errors (e.g. except InsufficientFundsError:) cleanly.

Re-raising exceptions with bare raise

Re-raising is using raise with no arguments inside an except block to pass the same error up after handling part of it.

Use raise without arguments inside an except block to re-trigger the current exception after logging it.

Python

1def connect_database():2    try:3        raise ConnectionError("DB unreachable")4    except ConnectionError as err:5        print(f"[LOG] Connection failure: {err}")6        # Re-raise to alert caller7        # raise

Example 3 · Log and pass the error up

Python

1def read_quantity(text):2    try:3        return int(text)4    except ValueError:5        print("Logging: bad quantity text")6        raise7
8try:9    read_quantity("two")10except ValueError as err:11    print(f"Caller handled: {err}")

Output

Logging: bad quantity text
Caller handled: invalid literal for int() with base 10: 'two'

What this code does

The inner except prints a log line, then a bare raise sends the same error up.
The outer try catches it and still sees the original message.

A bare raise re-emits the exact active exception with its original traceback intact.

Next step

Check your understanding

8 quick questions

1def set_discount(percent):2 if not 0 <= percent <= 100:3 raise ValueError("Discount must be between 0 and 100")4 return percent5 6try:7 set_discount(150)8except ValueError as err:9 print(f"Rejected: {err}")

1class InsufficientFundsError(Exception):2 """Raised when account balance is less than withdrawal amount."""3 pass4 5def withdraw(balance, amount):6 if amount > balance:7 raise InsufficientFundsError(f"Cannot withdraw {amount} from {balance}")8 return balance - amount

1class UserNotFoundError(Exception):2 pass3 4def find_user(user_id, database):5 if user_id not in database:6 raise UserNotFoundError(f"User with ID {user_id} does not exist")7 return database[user_id]8 9db = {101: "Aditi", 102: "Kabir"}10try:11 user = find_user(999, db)12except UserNotFoundError as err:13 print(f"Handled: {err}")

1def connect_database():2 try:3 raise ConnectionError("DB unreachable")4 except ConnectionError as err:5 print(f"[LOG] Connection failure: {err}")6 # Re-raise to alert caller7 # raise

1def read_quantity(text):2 try:3 return int(text)4 except ValueError:5 print("Logging: bad quantity text")6 raise7 8try:9 read_quantity("two")10except ValueError as err:11 print(f"Caller handled: {err}")